My invention relates to apparatus for automatically controlling the background brightness of copies produced by a document copier and, more particularly, to apparatus to be used with an electrostatic copier in which a uniformly charged photoconductive surface is selectively discharged to form an electrostatic latent image of the original document.
It is generally desirable for electrostatic or other document copiers to produce copies having a white background, regardless of the background density of the original document. If all originals are simply given a fixed exposure sufficient to ensure a white copy background for originals having a relatively dark background, such as multicolored forms, then the contrast between printed areas and background areas in copies of originals having relatively light backgrounds will be unnecessarily lowered. Although many copiers are provided with a manual brightness control, such a facility at best allows a trial-and-error approach resulting in many wasted copies. Other copiers avoid the problem of brightness control by using development systems sensitive only to image contrast. While such systems are suitable for copiers handling only line material such as ordinary typed or printed matter, they are not suitable for copiers that must often handle continuous-tone orginals or originals containing broad dark areas to be developed.
Several systems for automatically controlling copy brightness have been suggested. In one such system, described in U.S. Pat. No. 3,279,312 issued to Rogers, exposure is controlled by a signal provided by an optical detector disposed at a sensing station in advance of the imaging station. In another such system, described in U.S. Pat. No. 3,914,049 issued to Basu et al, an optical detector disposed in the optical path of the existing optical scanning system senses light from the original document during a prescan period in which the exposure lamp and optical scanning system are actuated. These systems, however, cannot readily distinguish between light and dark areas of documents to be copies and are thus subject to error.